Singapore is small, hot, humid and heavily populated — the 5.5 million residents of the tropical city-state live in less than 750 square kilometers of land. And population is expected to reach 6.9 million by 2030.

Despite these challenges, Singapore continues to be amongst the most livable and economically successful cities in the word, with a GDP equaling that of leading European countries.

With more than 50 percent of the world's population living in cities already, a figure projected to reach 70 percent by 2050, Singapore is setting trends for rapidly urbanizing countries worldwide. So how did they do that?

Have a plan: Concept plans have been in place since 1971, with long-term visions and predictions for the design of Singapore's infrastructure. Such long-term planning was crucial as the population was growing much faster than originally anticipated.

With limited land and no natural resources, planning for clean air, clean water and green cover was integral to Singapore's city planning. Decentralization of commercial hubs was also key in the country's urban design to reduce congestion and commuting time.

Don't waste your waste: With high-density living comes high-density waste. But Singapore has been organized with its refuse management systems, not only by collecting it efficiently but even employing it to make more land.

Waste is managed instead through regular incineration and the resulting ashes are combined with marine sand to extend Singapore's land mass. In addition, the country has set a precedent for water resources through its desalination plants and NEWater plants, where sewage is filtered to recycle it into drinking water.

See green with A/C: Being hot and humid, a lot of the city's energy expenditure goes towards cooling people down. Up to 60 percent of Singapore's electricity goes for cooling and lighting buildings. To overcome this overconsumption of energy, a project in now being projected that aims to dehumidify the air external to a building and then flow the resulting cool air over the façade of a building. This removes the need for air conditioning systems and saves up to 1/3 of the building’s space.

Go underground: When population increases and demand for land spikes accordingly, the tendency is to build upwards, as is the case for most cities worldwide. But with unique land restrictions compared to the rest of the world, Singapore has now begun to build downwards and is taking workplaces underground.

The design involves digging caverns into rocks more than 100 meters below the ground. Underground, everything including vibrations, temperature and humidity are much more stable. This is especially important for facilities such as power plants, water reservoirs and industrial settings in general.

Embrace technology: The technological development of Singapore cannot be forgotten as this aspect of city living has long been incorporated into the design of the city-state. The Singapore mass rapid transit (MRT) is considered among the best public transport systems in the world and intelligent buildings have been in use for more than a decade aiding movement and entertainment through public and work spaces.

Innovation has been at the root of the country's development, both for livability and sustainability. It is this emphasis on technology that has helped Singapore become one of the Greenest cities in the world.

Singapore is now exporting its expertise in urban planning to other cities in Asia where rapid urbanization is taking place —including the Tianjin Eco city, China and the new capital city in Andhra Pradesh, India — and paving the way for cities worldwide to ensure they build sustainably and improve their livability.